Accumulated Other Comprehensive Income (Loss) - Reclassifications (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Other income - net realized gains on sales of available for sale securities	$ 567	$ 706	$ 1,165	$ 1,703
Interest expense - subordinated debentures and term loans	1,840	1,786	3,657	3,571
Amortization of prior service costs	27,076	25,570	52,808	52,907
Income tax expense	(7,207)	(6,716)	(14,375)	(12,290)
Total reclassifications for the period, net of tax	264	252	537	690
Unrealized gains (losses) on available for sale securities
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period, net of tax	369	459	757	1,107
Unrealized gains (losses) on cash flow hedges
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period, net of tax	(163)	(207)	(337)	(417)
Unrealized Gains (Losses) on Defined Benefit Plans
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Total reclassifications for the period, net of tax			117
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) for the Period | Unrealized gains (losses) on available for sale securities
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Other income - net realized gains on sales of available for sale securities	567	706	1,165	1,703
Income tax expense	(198)	(247)	(408)	(596)
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) for the Period | Unrealized gains (losses) on cash flow hedges
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	88	111	182	225
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) for the Period | Unrealized gains (losses) on cash flow hedges | Interest rate contracts
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Interest expense - subordinated debentures and term loans	(251)	(318)	(519)	(642)
Amount Reclassified from Accumulated Other Comprehensive Income (Loss) for the Period | Unrealized Gains (Losses) on Defined Benefit Plans
Reclassification Adjustments out of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service costs	89		180
Income tax expense	(31)		(63)
Total reclassifications for the period, net of tax	$ 58	$ 0	$ 117	$ 0